Abstract: Based on 1980-1992 GMS T_BB data,we analyze the 13 years averaged T_BB power spectra in tropical regions as a function of time,and discuss the seasonal variations characteristics of low frequency oscillation period and its possibe mechanism.Results show that tropical low frequency oscillation periods have evident seasonal variations.The seasonal variations in northern hemisphere tropical regions are larger than those in adjacent equatorial regions and southern hemisphere tropical regions.In general,the low frequency oscillations periods in winter are longer than those in summer and fall.The seasonal variations characteristics of low frequency oscillation period can be explained by the mobile wave-CISK mechanism.When the heating strengthens,the phase speed of CISK wave gets larger and the low frequency oscillation period becomes shorter,v.v..